1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 63-33604 describes an optical encoder that detects a relative displacement between two members by using a three-grating principle. FIG. 11 illustrates the structure of an optical encoder 800 described in Japanese Unexamined Patent Application Publication No. 63-33604. The optical encoder 800 includes three gratings, which are a light source grating 120 disposed directly below a light source 110, a scale grating 850 disposed in a scale 840, and a light-receiving grating 151 disposed directly above a photodiode 152. A detection head 170, which includes the light source grating 120 and the light-receiving grating 151, moves relative to the scale 840; and the amount of movement of the detection head 170 is detected from a change in the intensity of a signal generated by the photodiode 152.
The light source grating 120 includes light-transmitting portions 121 and light-blocking portions 122, which are alternately arranged with a period P. The scale grating 850 includes light-transmitting portions 851 and light-blocking portions 852, which are alternately arranged with a period P. The light-receiving grating 151 includes light-transmitting portions 153 and light-blocking portions 154, which are alternately arranged with a period P. According to the three-grating principle, the light source grating 120, the scale grating 850, and the light-receiving grating 151 have the same grating pitch. Moreover, a gap between the light source grating 120 and the scale 840 and a gap between the scale 840 and the light-receiving grating 151 are the same as each other.
FIG. 12 is a cross-sectional view illustrating the light source grating 120, the scale grating 850, and the light-receiving grating 151. Cross-sections of light-blocking portions 122a to 122d, 852a to 852e, and 154a to 154d are hatched.
How the scale 840 moves and how interference fringes are generated due to the movement of the optical encoder 800 will be briefly described. To be specific, how a peak of signal intensity occurs every time the scale 840 moves by a half pitch according to the three-grating principle will be described. The description will be made with reference to FIGS. 12, 13, and 14. FIG. 12 illustrates an initial state. In the state shown in FIG. 12, the lines of the light source grating 120 and the scale grating 850 are aligned with each other. (Thus, there are paths through which 0-th order light can pass light-transmitting portions of the light source grating 120 and the scale grating 850.) The light-receiving grating 151 is disposed so that the lines thereof are aligned with those of the light source grating 120. Therefore, in the state shown in FIG. 12, the lines of the light source grating 120, the scale grating 850, and the light-receiving grating 151 are aligned with each other. In this state, the light-transmitting portions of the light source grating 120 will be referred to as 121a, 121b, 121c, . . . , from the right side. Likewise, the light-transmitting portions of the scale grating 850 will be referred to as 851a, 851b, 851c, . . . , from the right side. The light-transmitting portions of the light-receiving grating 151 will be referred to as 153a, 153b, 153c, . . . , from the right side.
In this state, bright interference fringes are formed at positions where light rays that have passed through light-transmitting portions of the light source grating 120 and the scale grating 850 and reached the light-receiving grating 151 along the same optical path length. For example, the optical path lengths of a light ray that passes through 121b, 851c, and 153c and a light ray that passes through 121b, 851b, and 153c are the same. Accordingly, a bright interference fringe is formed at the light-transmitting portion 153c of the light-receiving grating 151. Likewise, the optical path lengths of a light ray that passes through 121b, 851c, and 153b and a light ray that passes through 121b, 851a, and 153b are the same. Accordingly, a bright interference fringe is formed at the light-transmitting portion 153b of the light-receiving grating 151. As describe above, in the state shown in FIG. 12, light that has passed through the light source grating 120 and the scale grating 850 generates interference fringes including bright interference fringes arranged with a half-pitch period at the position of the light-receiving grating 151. Accordingly, all light rays forming the bright interference fringes pass through the light-transmitting portions of the light-receiving grating 151 and reach the photodiode 152. At this time, the signal intensity of the photodiode 152 is at a peak.
Next, suppose that the scale grating 850 gradually moves rightward from the state shown in FIG. 12. When the scale grating 850 moves, the positions of the interference fringes gradually change. As the positions of bright interference fringes become displaced from the light-transmitting portions of the light-receiving grating 151, the signal intensity of the photodiode 152 would gradually decrease. The signal intensity reaches a peak again when the scale 840 has moved by a half pitch as illustrated in FIG. 13.
As can be seen by tracing light rays in FIG. 13, the optical path lengths of a light ray that passes through 121b, 851d, and 153c and a light ray that passes through 121b, 851b, and 153c are the same. Accordingly, a bright interference fringe is formed at the light-transmitting portion 153c of the light-receiving grating 151. (Recall that, also in the state shown in FIG. 12, a bright interference fringe is formed at the light-transmitting portion 153c of the light-receiving grating 151.) Likewise, for example, the optical path lengths of a light ray that passes through 121b, 851c, and 153b and a light ray that passes through 121b, 851b, and 153b are the same. Accordingly, a bright interference fringe is formed at the light-transmitting portion 153b of the light-receiving grating 151.
As described above, in the state shown in FIG. 13, light that has passed through the light source grating 120 and the scale grating 850 form interference fringes including bright interference fringes arranged at a half-pitch period at the position of the light-receiving grating 151. These interference fringes are the same as those formed in the state shown in FIG. 12 (before the scale grating 850 moves by a half pitch).
FIG. 14 illustrates a state in which the scale grating 850 has moved further by a half pitch, which is substantially the same as the state shown in FIG. 12. Accordingly, interference fringes formed in the state shown in FIG. 14 are the same as those of FIG. 12.
FIG. 15 is a graph representing a change in the detection signal that occurs when the scale grating 850 moves. The detection signal reaches a peak every time the scale grating 850 moves by a half pitch. Although it may not be possible to make the line spacing of each of the light source grating 120, the scale grating 850, and the light-receiving grating 151 smaller than the pitch P due to limitation on manufacturing technology, an encoder including these grating has a resolution of a half pitch (P/2). This a great advantage of an encoder using the three-grating principle.
Moreover, an encoder using the three-grating principle has the following advantage. Because the same change in the signal intensity is repeated with a certain period (every time the scale grating 850 moves by a half pitch), interpolation of dividing one period of the signal can be performed so as to detect a displacement of the scale grating 850 smaller than the signal period (half pitch). Note that this interpolation, in which a half period P/2 is interpolated, provides a resolution that is twice a resolution obtained by performing interpolation in which a period P is interpolated.
As described above, according to the three-grating principle, a peak of the detection signal can be obtained every time the scale grating 850 moves by a half pitch. To be more specific, the signal intensity at peaks XII and XIV slightly differs from that at a peak XIII. In FIG. 15, the peak XII corresponds to the state shown in FIG. 12, the peak XIII corresponds to the state shown in FIG. 13, and the peak XIV corresponds to the state shown in FIG. 14. Although the peak of the signal occurs every time the scale grating 850 moves by a half pitch, the signals obtained at every half pitch differ from each other. This means that it is not possible to correctly perform interpolation, which assumes that the same signal can be obtained at every certain period (every time the scale grating 850 moves by a half pitch).
It is certain that the peak XII (in the case shown in FIG. 12) is the same as the peak XIV (in the case shown in FIG. 14), which is formed when the scale grating 850 moves by one pitch from the state shown in FIG. 12. Accordingly, in order to perform interpolation correctly, it is necessary to interpolate the period P. If interpolation were performed with a period P/2, an interpolation error would be unavoidably generated. (The interpolation error would have a period P.) It is meaningless to perform interpolation of the period P/2, for obtaining a high resolution, only to generate an unavoidable error with the period P.
The inventors noticed this problem and investigated the cause of the problem. As a result, the inventors found that, even though positions of the interference fringes generated in the case of FIG. 12 (FIG. 14) are the same as those of the case of FIG. 13, the characteristics of the optical paths in these cases differ from each other.